This invention relates to angular displacement sensor devices, such as level indicators, tilt indicators and accelerometers.
Sensors are known which are used for measuring the angular displacement of a reference planar surface from a reference axis. Such devices are used to measure the tilt of an object from vertical, the magnitude and direction of small vibrations, e.g. seismic vibrations, and the like. Some devices have employed electromagnetic radiation and photosensitive detectors for measuring the amount, or angle of, radiation reflected from a surface, such as a mirror, which maintains a fixed (e.g., horizontal) attitude when the sensor housing is angularly displaced with respect to a reference axis or plane. Other known devices have employed a fluid surface as the reflecting surface which provides the reference plane for measuring the angular displacement of the sensor housing, while still other known devices of this type have employed a transmittive fluid column whose length changes in a manner proportional to the degree of tilt of the sensor casing.
Known sensors of the above type suffer from several limitations. Some sensors provide a lower limit of resolution which is too large to provide meaningful data for small vibrations or angular displacements, such as seismic microvibrations. Others are sensitive to both linear accelerations and/or linear motion, as well as angular motion, which can be separated from the significant data, if at all, only by the use of complicated and expensive electronic circuits. Perhaps the most significant problem, however, with respect to prior art sensors of the above type is that of obtaining calibrated data over wide temperature variations. All mechanical, optical and electrical elements possess important characteristics which vary with the ambient temperature to which they are subjected. Mechanical elements, for example (and optical elements as well), all possess a finite coefficient of thermal expansion which affects translatory and rotary motion upon which sensors depend for accurate results. Similarly, electrical elements possess temperature dependent characteristics, such as temperature coefficient of resistance, temperature coefficient of conductance, and the dielectric constant which alter the absolute magnitude of output signals generated by electrical circuitry employing such elements. In general, the finer the lower limit of the resolution desired for a given sensor, the more expensive the cost of manufacturing a suitable sensor, which, in many cases, provides only marginal accuracy.
Efforts in the past to design sensors devoid of the above limitations have not met with success.